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11.3. COMBINING SEARCHING AND EXTRACTING
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When we run the program, we see the data nicely filtered to show only the lines
we are looking for.
X-DSPAM-Confidence: 0.8475
X-DSPAM-Probability: 0.0000
X-DSPAM-Confidence: 0.6178
X-DSPAM-Probability: 0.0000
But now we have to solve the problem of extracting the numbers. While it would
be simple enough to use split, we can use another feature of regular expressions
to both search and parse the line at the same time.
Parentheses are another special character in regular expressions. When you add
parentheses to a regular expression, they are ignored when matching the string.
But when you are using findall(), parentheses indicate that while you want the
whole expression to match, you only are interested in extracting a portion of the
substring that matches the regular expression.
So we make the following change to our program:
# Search for lines that start with 'X' followed by any
# non whitespace characters and ':' followed by a space
# and any number. The number can include a decimal.
# Then print the number if it is greater than zero.
import re
hand = open('mbox-short.txt')
for line in hand:
line = line.rstrip()
x = re.findall('^X\S*: ([0-9.]+)', line)
if len(x) > 0:
print(x)
# Code: http://www.py4e.com/code3/re11.py
Instead of calling search(), we add parentheses around the part of the regular
expression that represents the floating-point number to indicate we only want
findall() to give us back the floating-point number portion of the matching
string.
The output from this program is as follows:
['0.8475']
['0.0000']
['0.6178']
['0.0000']
['0.6961']
['0.0000']
..
The numbers are still in a list and need to be converted from strings to floating
point, but we have used the power of regular expressions to both search and extract
the information we found interesting.